1. Field of the Invention
The invention relates to a method of producing a glass layer and somewhat more particularly to a method of producing a glass layer on an interior surface of a hollow body by thermal decomposition of a reactive gas mixture suitable for glass synthesis.
2. Prior Art
Methods of producing a glass layer on an inner surface of a hollow body, such as a tube, wherein a suitable reactive gas mixture for glass synthesis is introduced into a hot region within the interior of such body and generated by external heating of the body so that a chemical reaction occurs at the hot region and a material having the composition of glass is generated from the gas mixture and is deposited on the inner surface of the body as a layer which is then melted and fused to form a clear glass film, are known.
These type of production methods are used, for example, to produce high purity glass preforms useful in production of glass fiber lightwave guides and are sometimes referred to as CVD (chemical vapor deposition) techniques (see, for example, Survey Papers, XIth Intern. Congr. Glass, Vol. II, Prague, 1977, pages 114-157; Chem. Eng. Techn., Vol. 51, 1979, pages 612-627 and Appl. Opt., Vol. 18, 1979, pages 3684-3693 wherein there is presented a detailed comparison of different known production techniques).
In general, with the CVD technique, a reactive gas mixture suitable for glass synthesis is introduced into a hollow glass tube, generally composed of high purity quartz glass. The reactive gas mixture is comprised, in general, of gaseous halides and oxygen. The glass tube is heated on its exterior surface by a suitable furnace or burner, for example, a hydrogen burner, so that a localized hot region is generated in the interior of the tube which also encompasses a corresponding hot interior surface of such tube. Within this hot region, the introduced halides are combusted. The oxides or oxide mixtures which are so-generated collect or are deposited as a powdery or glassy material on the inner surface of the tube as a layer which is then melted into a clear glass film in the hot region. As a rule, the burners used only heat a relatively short piece of a tube over its entire circumference. In coating longer lengths of a tube, a burner is moved relative to the tube so that the local hot region in the interior of the tube gradually travels along the length of the tube and the inner surface or wall along this length is uniformly coated with a layer of synthetic glass. By repeating this coating process a plurality of times, several glass layers can be generated one on top of another on the inner surface of a tube. A tube coated in this manner is then shaped into a solid rod and thus a blank workpiece or perform for glass fiber lightwave guides is attained. Glass fibers are pulled from this rod-shaped workpiece in a conventional manner to form lightwave guides.
The CVD technique produces coated glass tubes or preforms of especially high purity from which glass fiber lightwave guides can be attained and which exhibit excellent quality, particularly extremely low attenuation with simultaneously high band widths. Tubes heated by flames yield lightwave guides having particularly low attenuation values.
However, a limitation of the CVD technique is that it is capable of only a relatively slow glass formation rate. Generally, it is slower by a factor of 5 to 15 relative to other known methods and thus leads to correspondingly low production rates. A reason for such relatively low glass formation rate is that with the CVD method, the layer of material must be deposited on an inner wall and substantially simultaneously be melted or sintered into clear glass. The sintering velocity thus determines the glass formation rate. However, the permissible sintering velocity is limited. As experience has shown, sintering too fast leads to bubbled glass.
A suggested technique for increasing sintering velocity and thus glass formation rate comprises, according to Appl. Phys. Lett., Vol. 31, 1977, pages 174-176, adding helium to the gas mixture introduced into a tube. With this technique, five times thicker powder coatings should be meltable, relative to a coating meltable with a customary gas mixture.